Change default reference angle for Weld joints

Weld joints in Love didn't have a reference angle as argument before 0.10.2, which meant it defaulted to Box2D value of (body2 angle - body1 angle).
After the argument was added, however, it started to default to 0 rads, which breaks games written before 0.10.2 that use Weld joints. Moreover, Weld joints work more intuitively with the default angle that Box2D originally uses.